1. Field of the Invention
The invention relates to a calcining process for removing arsenic from green coke made from shale oil containing at least about 10 ppm arsenic.
2. Description of the Prior Art
Oil obtained from oil shale typically has a relatively high arsenic content compared with petroleum oil. The presence of arsenic in crude shale oil makes it an undesirable feedstock for a conventional refinery because organoarsenic compounds form complexes with many of the metals used in refining catalysts, thereby poisoning the catalysts. Also, arsenic is a potential health hazard and deposits easily causing scaling and plugging of lines, tubes, packings, trays and other equipment. In view of this many techniques have been devised to remove arsenic from shale oil. For instance, U.S. Pat. No. 4,029,571 describes a thermolytic process in which the shale oil is heated under conditions that maintain most of the oil in a liquid phase but cause the formation of a small amount of precipitate in which the arsenic is concentrated. The precipitate is removed from the fluid by conventional solid-liquid separation techniques.
Crude shale oil is also often upgraded to improve its pour point and viscosity. Various conventional upgrading processes, including coking, have been suggested or used. In this regard pages 125-132 of the Synthetic Fuels Data Handbook, Cameron Engineers, Inc., 1975 describes coking processes for shale oil and reports analyses of the distillate and coke products from coking tests on shale oil feedstocks. The arsenic content of the coke is not indicated and there is no mention of further treatment of the coke.
Several patents describe processes for calcining green coke made from petroleum oil for the purpose of removing volatile hydrocarbons and/or sulfur from the green coke. U.S. Pat. No. 3,369,871 describes such a process in which green petroleum coke is first treated with oxygen to oxidize sulfur in the coke and then calcined at elevated temperatures in a non-oxidizing atmosphere. U.S. Pat. No. 3,933,596 describes another such process in which the green coke is mixed with a small amount of sodium cabonate and heated followed by contact with a desulfuring gas at elevated temperatures. Other processes for calcining petroleum oil coke are described in U.S. Pat. Nos. 4,100,265 and 4,146,434. None of these patents mention coke derived from shale oil.